netdev - Re: [PATCH net-next 01/12] tipc: change socket buffer overflow control to respect sk

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Date:	Mon, 3 Jun 2013 09:16:24 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Ying Xue <ying.xue@...driver.com>
Cc:	Paul Gortmaker <paul.gortmaker@...driver.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	Jon Maloy <jon.maloy@...csson.com>,
	Erik Hugne <erik.hugne@...csson.com>
Subject: Re: [PATCH net-next 01/12] tipc: change socket buffer overflow
 control to respect sk_rcvbuf

On Mon, Jun 03, 2013 at 05:55:06PM +0800, Ying Xue wrote:
> On 05/31/2013 09:36 PM, Neil Horman wrote:
> > On Thu, May 30, 2013 at 03:36:06PM -0400, Paul Gortmaker wrote:
> >> From: Jon Maloy <jon.maloy@...csson.com>
> >>
> >> As per feedback from the netdev community, we change the buffer
> >> overflow protection algorithm in receiving sockets so that it
> >> always respects the nominal upper limit set in sk_rcvbuf.
> >>
> >> Instead of scaling up from a small sk_rcvbuf value, which leads to
> >> violation of the configured sk_rcvbuf limit, we now calculate the
> >> weighted per-message limit by scaling down from a much bigger value,
> >> still in the same field, according to the importance priority of the
> >> received message.
> >>
> >> Cc: Neil Horman <nhorman@...driver.com>
> >> Signed-off-by: Jon Maloy <jon.maloy@...csson.com>
> >> Signed-off-by: Paul Gortmaker <paul.gortmaker@...driver.com>
> >> ---
> >>  net/tipc/socket.c | 13 +++++++------
> >>  1 file changed, 7 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/net/tipc/socket.c b/net/tipc/socket.c
> >> index 515ce38..2dfabc7 100644
> >> --- a/net/tipc/socket.c
> >> +++ b/net/tipc/socket.c
> >> @@ -1,7 +1,7 @@
> >>  /*
> >>   * net/tipc/socket.c: TIPC socket API
> >>   *
> >> - * Copyright (c) 2001-2007, 2012 Ericsson AB
> >> + * Copyright (c) 2001-2007, 2012-2013, Ericsson AB
> >>   * Copyright (c) 2004-2008, 2010-2012, Wind River Systems
> >>   * All rights reserved.
> >>   *
> >> @@ -203,6 +203,7 @@ static int tipc_create(struct net *net, struct socket *sock, int protocol,
> >>  
> >>  	sock_init_data(sock, sk);
> >>  	sk->sk_backlog_rcv = backlog_rcv;
> >> +	sk->sk_rcvbuf = CONN_OVERLOAD_LIMIT;
> > The last time Jon and I discussed this, I thought the consensus was to export
> > sk_rcvbuf via its own sysctl, or tie it to sysctl_rmem (while requiring a
> > protocol specific minimum on top of that), so administrators on memory
> > constrained systems didn't wonder why their sysctl changes weren't being
> > honored.
> 
> Yes, your suggestion is reasonable, and I prefer to involve
> net.tipc.sysctl_rmem. But I have one question about it:
> 
> As you suggested as belows, the default value of sk->sk_rcvbuf is set to
> sk->sk_rcvbuf >> 4 << msg_importance(TIPC_CRITICAL_IMPORTANCE), that is,
> sk->sk_rcvbuf is about 32MB.
> 
> However, please see below code:
> 
> int sock_setsockopt()
> {
> ...
> 	        case SO_RCVBUF:
>                 /* Don't error on this BSD doesn't and if you think
>                  * about it this is right. Otherwise apps have to
>                  * play 'guess the biggest size' games. RCVBUF/SNDBUF
>                  * are treated in BSD as hints
>                  */
>                 val = min_t(u32, val, sysctl_rmem_max);
> set_rcvbuf:
>                 sk->sk_userlocks |= SOCK_RCVBUF_LOCK;
>                 /*
>                  * We double it on the way in to account for
>                  * "struct sk_buff" etc. overhead.   Applications
>                  * assume that the SO_RCVBUF setting they make will
>                  * allow that much actual data to be received on that
>                  * socket.
>                  *
>                  * Applications are unaware that "struct sk_buff" and
>                  * other overheads allocate from the receive buffer
>                  * during socket buffer allocation.
>                  *
>                  * And after considering the possible alternatives,
>                  * returning the value we actually used in getsockopt
>                  * is the most desirable behavior.
>                  */
>                 sk->sk_rcvbuf = max_t(u32, val * 2, SOCK_MIN_RCVBUF);
>                 break;
> ...
> }
> 
> From above logic of setting sk->sk_rcvbuf with SO_RCVBUF, it only
> permits the maximum value of sk->sk_rcvbuf to sysctl_rmem_max * 2(ie,
> about 400KB normally).
> 
> So, even if the default value of sk->sk_rcvbuf is set to 32MB with
> net.tipc.sysctl_rmem, a bit smaller value than the default value can
> never be set to sk->sk_rcvbuf successfully with SO_RCVBUF option.
> 
> How can we avoid the limit?
> 
By administratively adjusting sysctl_rmem_max to be a sufficiently large value
such that using SO_RCVBUF won't be clamed to a lower limit.

If you don't want to force users to have to manually adjust the sysctl, there
might be support for you to automatically update sysctl_rmem_max in your
tipc_init routine, and print an informational message indicating that tipc
requires the additional space (although I still maintain its not strictly
needed, but thats another argument).

Neil

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